The present disclosure relates to wind turbines. More particularly, it relates to wind turbines with an increased average power output.
Due to limited availability of suitable areas for wind turbines on land, the concept of off-shore wind energy production has gained importance in recent years. In shallow costal waters, wind turbines are usually mounted on concrete foundations on the bed of the body of water, whereas in deeper waters, the turbines may float in the water, and may be fixed via chains, cables, lines or moorings to the bed of the body of water, for instance to the sea bed or the ground of a lake.
Operators of devices located at sea, or more generally in waters, face some problems not encountered on land. One of these factors is the presence of swell, respectively waves, which may affect the operational safety if they exceed certain amplitudes, especially in mooring-mounted wind turbines. On the other hand, the wind force often exceeds the limit at which the wind turbine has its maximum power output. If the wind speed increases beyond this level, the wind turbine power output does not increase any more. Instead, in order to avoid overload of the system, the pitch angle of the rotor blades is typically reduced by the operator or automatically. This leads to decreasing aerodynamic properties of the rotor and thus to a reduced electrical power output of the wind turbine. Hence, above a certain wind speed, the electrical power output is not increasing with wind speed, but decreasing, which is undesirable.
Further, it is known that wind speed increases, up to a certain altitude, with the distance from the ground or the sea. A higher wind turbine thus principally results in a higher average electrical power output (AEP) during wind speeds in a regime sufficiently far removed from the wind speed at maximum power output. However, off-shore wind turbines must also withstand rough weather conditions. The higher a turbine, the higher are also forces which have to be accounted for by constructional measures, which leads to the undesirable increase of costs. At the same time, if the wind turbine is higher, the average time during which the wind speed exceeds the above-mentioned maximum power level of the turbine also increases. Thus, the gain in power output by the increased height is partly compensated by the fact that the wind turbine will on average also run with lower efficiency for a longer time, caused by elongated periods with wind speeds exceeding the speed at which the turbine reaches its nominal power level.
In light of the above, it is desirable to have a wind turbine, particularly, but not necessarily for off-shore use, which takes advantage of an increased height, but at the same time avoids the known disadvantages of an increased height.